1. Field of the Invention
This invention relates to an improvement in the process of making heat shrinkable film and to the resultant film of certain linear low density polyethylene ("LLDPE") polymers.
2. Description of Related Art
Heat shrinkable polymer film is typically made by extruding polymer from a melt into a film, followed by orienting the film by stretching it under temperature conditions where molecular orientation of the film occurs and the film does not tear. The film is then cooled in the stretched state, and upon subsequent heating, the film shrinks in seeking to cover its original dimensional state.
Irradiation of the film has been used prior to stretching to crosslink the film (polymer), thereby increasing resistance to tearing during stretching, but this has the twofold disadvantage of the increased cost of radiation and the inability to recycle through melt processing of any scrap generated in the heat shrinkable film manufacture.
The process described in U.S. Pat. No. 3,141,912 for making heat shrinkable film has achieved commercial utility for certain polymers without the need for irradiation of the film prior to stretching. In the continuous process of this patent, the polymer is extruded as a film in tubular form, the tube is quenched to a temperature below the orientation temperature range and is reheated to the orientation temperature range, followed by biaxially stretching of the film of the tube while within this temperature range. The biaxial stretch is done by (a) using internal gas pressure to expand the diameter of the tube to form a large "bubble" and (b) advancing the expanded tube at a faster rate than the extrusion rate, so as to obtain transverse and machine directions of orientation, respectively. Usually, the stretch is at least 3.times. in each direction. The film is then cooled and rolled up in the cooled state so as to retain the property of heat shrinkability.
U.S. Pat. No. 4,820,557 discloses the manufacture of multilayer heat shrinkable film in which one of the layers is a linear copolymer of ethylene with either 1-octene or 1-hexene, the linear copolymer having specified melt flow ratios which indicate a narrow molecular weight distribution and a density of 0.935 g/cc or less. The layer is also disclosed as being a blend of this copolymer with a large number of other polymers, including low density polyethylene ("LDPE"). The manufacture of the heat shrinkable film in this patent, however, is disclosed to involve radiation of the film prior to stretching.
Japanese Pat. Publication 60-257,232 (1985) discloses the compounding of LDPE and LLDPE together with a free-radical generator to cause the polymers to crosslink in the film extruder. The film is produced on a conventional blown film process in which the bubble is blown at or near the polymer's melting point. The film has little to no orientation and is not heat shrinkable. Bags of the resultant film are disclosed to have good heat seal strength.
U.S. Pat. No. 4,597,920 discloses the manufacture of heat shrinkable film by the process of U.S. Pat. No. 3,141,912 wherein the linear copolymer is of ethylene with at least one C.sub.8 -C.sub.18 alpha-olefin, the copolymer having a melt index of 0.1 to 4.0 g/10 min, density of 0.900 to 0.940 g/cc, a broad molecular weight distribution as indicated by the stress exponent being above 1.3, and having two distinct crystallite melting regions (melting points) at least 10.degree. C. apart. The patent also discloses that irradiation of extruded film prior to stretching is optional. In fact, the presence of the two distinct melting regions in the copolymer has enabled the heat shrinkable film of ethylene/1-octene copolymer to be made on a commercial basis without crosslinking prior to stretching. This commercial film is called Clysar.RTM. LLP shrink film.
Included in this patent is the disclosure in Table IV of blending increasing amounts of low density branched polyethylene with ethylene/1-octene copolymer; a decrease in film shrink force is the reported result. This aspect of the disclosure did not progress beyond the laboratory work reported wherein the stretching of film of the blends was done on a laboratory stretcher in an operation which was discontinuous from the film formation by melt pressing.
Despite the commercial success of the heat shrinkable film of ethylene/1-octene copolymer of U.S. Pat. No. 4,597,920, it has been desired to increase the economy of manufacture of this film without undertaking the expense of irradiation of the film prior to stretching and without being subject to the penalty of having non-melt processible scrap from the film manufacture. One way of economizing would be to increase the rate of production of the film on the same manufacturing equipment. The rate of production of this heat shrinkable film has been limited to the difficulty of achieving the temperature control required for orienting the film within the stretching zone of the manufacturing machine. The film extrusion rate has to be adjusted so that the machine direction stretch desired can be obtained with the "bubble" being stably positioned in the stretching zone of the manufacturing equipment, including the shape of the bubble being symmetrical, indicating that uniform stretching of the film is present. This condition of bubble stability provides the condition for biaxial orientation without causing tearing of the film during stretching. If the extrusion rate could be increased without causing tearing and without reducing the machine direction stretch, then a greater amount of heat shrinkable film would be obtained from the same manufacturing equipment. In the case of LLDPE wherein the copolymer is ethylene/1-hexene, the problem of producing heat shrinkable film without crosslinking prior to stretching is different. Such production is hardly possible at all. Thus, the production of heat shrinkable film from this copolymer without crosslinking prior to stretching does not appear to be reported in the prior art. The temperature required for orientation is so close to the melting point of the copolymer that film tearing occurs at even low extrusion rates. Because of this close proximity between orientation temperature and melting point, the strength of the copolymer is very low, which is responsible for the propensity of the film to orient non-uniformly and tear. It was for this reason that U.S. Pat. No. 4,597,920 required the alpha-olefin comonomer with ethylene to contain at least 8 carbon atoms, thereby excluding the disclosure of 1-hexene.
Furthermore, U.S. Pat. No. 4,597,920 requires LLDPE octene copolymer must have two distinct crystalline melting regions.